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Biology - Histology The Human Heart. Abstract: Dorland's Illustrated Medical Dictionary defines the heart as "the viscus of cardiac muscle that maintains the circulation of the blood". It is divided into four cavities; two atria and two ventricles. The left atrium receives oxygenated blood from the lungs. From there the blood passes to the left ventricle, which forces it via the aorta, through the arteries to supply the tissues of the body. The right atrium receives the blood after it has passed through the tissues and has given up much of its oxygen. The blood then passes through the right ventricle into the lungs where it gets oxygenated. There are four major valves in the heart; the left atrioventricular valve (also known as the mitral or bicuspid valve), the right atrioventricular valve (tricuspid), aortic valve, and the pulmonary valve. The heart tissue itself is nourished by the blood in the coronary arteries.2 Position of the Heart Within the Body: The heart is placed obliquely in the chest. The two atria are directed upwards and backwards to the right and are at the level of the fifth through the eight dorsal vertebrae. The apex of the heart points downwards and forwards to the left and corresponds to the interspace between the fifth and sixth ribs, two inches below the left nipple. Its atrial border corresponds to a line drawn across the sternum on a level with the upper border of the third costal cartilage. Its lower border (apex) corresponds to a line drawn across the lower end of the same bone, near the xiphoid process. Its upper surface is rounded and convex, directed upwards and forwards, and formed mainly by the right ventricle and part of the left ventricle. The posterior surface of the heart is flattened and rests upon the diaphragm muscle. Of its two borders, the right is the longest and thinnest, the left is shorter but thicker and round. Size: In an adult, the heart measures about five inches in length, three and a half inches in the broadest part of its transverse diameter, and two and a half inches in its antero-posterior. The average weight in the male varies from ten to twelve ounces. In the female, the average weight is eight to ten ounces. The heart will continue to grow in size up to an advanced period of life. This growth is more obvious in men than in women.3 Circulation of Blood in an Adult: The heart is subdivided by a longitudinal muscular septum into two lateral halves which are named right and left according to their position. A transverse muscle divides each half into two cavities. The upper cavity on each side is called the atria/auricle, and the lower side is called the ventricle. The right atrium and ventricle form the venous side of the heart. Dark venous blood is pumped into the right atrium from the entire body by the superior (SVC) and inferior vena cava (SVC), and the coronary sinus. From the right atrium, the blood passes into the right ventricle and from the right ventricle, through the pulmonary artery into the lungs.3 Once the blood becomes oxygenated/arterialized by its passage through the lungs, it is returned to the left side of the heart by the pulmonary veins which open into the left atrium. From the left atrium, the blood passes into the left ventricle where it is distributed by the aorta and its subdivisions through the entire body. Morphology of Each Heart Chamber: The right atrium is a little longer than the left. Its walls are also somewhat thinner than the left. The right atrium is capable of containing about two ounces of fluid. It consists of two parts, a principle cavity/sinus, and an appendix auriculae. The sinus is a large quadrilateral-shaped cavity located between the IVC and the SVC. Its walls are extremely thin and are connected on the lower surface with the right ventricle and internally with the left atrium. The rest of the right atrium is free and unattached. The appendix auricle is a small conical muscular pouch. It projects from the sinus forwards and to the left side, where it overlaps the root of the pulmonary artery.6 There are four main openings into the right atrium; the SVC, IVC, coronary sinus, and the atriculo-ventricular opening. The larger IVC returns blood from the lower half of the body and opens into the lowest part of the right atrium, near the septum. The smaller SVC returns blood from the upper half of the body and opens into the upper and front part of the right atrium. The coronary sinus opens into the right atrium between the IVC and auriculo-ventricular opening. It returns blood from the cardiac muscle of the heart and is protected by a semicircular fold of the lining membrane of the atrium, called the coronary valve. The auriculo-ventricular opening is the large oval aperture of communication between the right atrium and ventricle. There are two main valves located within the right atrium; the Eustachian valve and the coronary valve.3 The Eustachian valve is located between the anterior margin of the IVC and the auricule-ventricular orifice. It is semilunar in form. The coronary valve is a semicircular fold of the lining membrane of the right atrium, protecting the orifice of the coronary sinus. The right ventricle is triangular-shaped and extends from the right atrium to near the apex. Its anterior surface is rounded and convex and forms the larger part of the front of the heart. Its posterior surface is flattened, rests on the diaphragm muscle, and forms only a small part of this surface. Its inner wall is formed by the partition between the two ventricles, the septum, and bulges into the cavity of the right ventricle. Superiorly, the ventricle forms a conical structure called the infundibulum from which the pulmonary artery arises. The walls of the right ventricle are thinner than those of the left ventricle. The thickest part of the wall is at the base and it gradually becomes thinner towards the apex. The cavity can contain up to two ounces of fluid. There are two openings in the right ventricle; the auriculo-ventricular opening and the opening of the pulmonary artery. The auriculo-ventricular opening is the large oval opening between the right atrium and the right ventricle. The opening is about an inch in diameter. It is surrounded by a fibrous ring, covered by the lining membrane of the heart (endocardium), and is larger than the opening between the left atrium and the left ventricle. It is protected by the tricuspid valve. The opening of the pulmonary artery is round and is situated at the top of the conus arteriosus, close to the septum. It is on the left side and is in front of the auriculo-ventricular opening. It is protected by the semilunar valves.3 There are two main valves associated with the right ventricle; the tricuspid valve and the semilunar valves. The tricuspid valve consists of three segments of a triangular shape, formed by the lining membrane of the heart (endocardium). They are strengthened by a layer of fibrous tissue and muscular fibers.1 These segments are connected by their bases to the auriculo-ventricular orifice, and by their sides with one another, so as to form a continuous membrane which is attached around the margin of the auriculo-ventricular opening. Their free margin and ventricular surfaces are attached to many delicate tendinous cords called chordae tendinae. The central part of each valve segment is thick and strong while the lateral margins are thin and indented. The chordae tendinae are connected with the adjacent margins of the main segment of the valves. The semilunar valves guard the opening of the pulmonary artery. They consist of three semicircular folds formed by the endothelial lining of the heart and are strengthened by fibrous tissue. They are attached by their convex margins to the wall of the artery at its junction with the ventricle. The straight borders of the valve are unattached and are directed upwards in the course of the vessel, against the sides of which they are pressed during the passage of blood along its canal. The free margin of each valve is somewhat thicker than the rest of the valve and is strengthened by a bundle of tendinous fibers. During the passage of blood along the pulmonary artery, these valves are pressed against the sides of its cylinder. During ventricular diastole (rest), when the current of blood along the pulmonary artery is checked and partly thrown back by its elastic walls, these valves become immediately expanded and close the entrance of the tube. 3 The left atrium is smaller but thicker than the right atrium. It consists of two parts; a principle cavity/sinus and an appendix auriculae. The sinus is cuboidal in form and is covered in the front by the pulmonary artery and the aorta. Internally, it is separated from the right atrium by the septum auricularum. Behind the sinus on each side, it receives the pulmonary veins. The appendix auriculae in the left atrium is narrower and more curved than the same structure in the right atrium. Its margins are more deeply indented, presenting a kind of foliated appearance. Its direction is forwards towards the right side, overlapping the root of the pulmonary artery. There are two main openings in the left atrium; the openings of the four pulmonary veins and the atrial-ventricular opening. Two of the four pulmonary veins open into the right side of the atrium and two open into the left side. The two veins on the left exit into the atrium through a common opening. None of the pulmonary veins have valves. The atrial-ventricular opening is the large oval opening of blood flow between the atrium and the ventricle. It is smaller than the same opening between the right atrium and ventricle.3 The left ventricle is longer and more conical shaped than the right ventricle. It forms a small part of the left side of the anterior surface of the heart and a large portion of the posterior surface. It also forms the apex of the heart because it extends beyond the right ventricle. Its walls are nearly twice as thick as those of the right ventricle. They are thickest in the broadest part of the ventricle, becoming gradually thinner towards the base and also towards the apex, which is the thinnest part of the left ventricle. There are two main openings in the left ventricle; the atrial-ventricular opening and the aortic opening. The atrial-ventricular opening is located behind and to the left side of the aortic opening. The opening is a little smaller than the same opening between the right atrium and ventricle. Its position corresponds to the center of the sternum. It is surrounded by a dense fibrous ring and is covered by the lining membrane of the heart and is protected by the mitral valve. The circular aortic opening is located in front of and to the right side of the atrial-ventricular opening from which it is separated by one of the segments of the mitral valve. The opening is protected by the semilunar valves. There are two valves located within the left ventricle; the mitral valve and the semilunar valve. The mitral valve is attached to the circumference of the atrial-ventricular opening in the same way that the tricuspid valve is attached on the opposite side of the heart. The valve contains a few muscular fibers, is strengthened by fibrous tissue, and is formed by the lining of the heart (endocardium). It is larger, thicker, and stronger than the tricuspid, and consists of two segments of unequal size. The mitral valves are connected to many chordae tendonae. Their attachment is the same as on the right side except they are thicker, stronger, and less numerous. The semilunar valves surround the aortic opening. They are similar in structure and mode of attachment to those of the pulmonary artery. However, they are larger, thicker, and stronger than those of the right side. Between each valve and the cylinder of the aorta is a deep depression called the sinuses of Valsalva. The depressions are larger than those at the root of the pulmonary artery.3 Figure 1: a. Cross sectional view of the heart. b. Top view of the heart showing the four valves Histology of the Layers of the Heart: The heart and its vessels are surrounded by a conical membranous sac called the pericardium. The pericardial sac is composed of two layers; the parietal pericardium and the visceral pericardium with the space in-between the two being called the pericardial cavity. The parietal pericardium is composed primarily of compact fibrocollagenous tissue along with elastic tissue. It is a fibrous membrane of loose irregular connective tissue that is lined internally by a mesothelium which is essentially simple squamous epithelium. The visceral pericardium forms the internal lining of the pericardium and reflects over the outer surface of the heart. This reflection forms the outer layer of the epicardium. The visceral pericardium is also composed of compact fibrocollagenous tissue with elastic tissue but, is smooth mesothelium. The pericardial cavity is located between the parietal and visceral pericardium and contains small amounts of serous fluid. The heart tissue itself can be subdivided into three layers; (from the outside in) epicardium, myocardium, and endocardium. The epicardium is the outermost layer of the heart and consists of a loose connective tissue of fibroblasts, collagen fibers, and adipose tissue. It contains a stroma which houses coronary arteries and veins that are surrounded by a layer of fat. These coronary branches penetrate the myocardium. The myocardium contains the main muscle mass of the heart and is composed primarily of striated muscle cells. Each of the cardiac muscle cells contain one central elongated nucleus with some central euchromatin and some peripheral heterochromatin. The two atria have a very thin myocardial layer which increases greatly in thickness as you go from the atria to the right ventricle and into the left ventricle. The outer surface of the myocardium, next to the epicardium, is not composed of smooth muscle but is very smooth in texture. The inner surface of the myocardium is rough and is raised into trabeculations. The ventricular papillary muscles, which are for the attachment of the chordae tendinae, are extensions of the myocardium even though they are covered by endocardium. The outer layer of the myocardium is superficial bulbospiral and swirls around the ventricle in a clockwise fashion. The middle layer is circular muscles that are the ventricular constrictors. The inner layer, which is deep bulbospiral, swirls around the ventricle in a counterclockwise fashion. The layer underneath the myocardium is known as the enodcardium. It contains a continuous smooth endothelial layer that covers all the inner surfaces of the heart, including the valves. The outer layer of the endocardium, underneath the myocardium, is irregularly arranged collagenous fibers that may contain Purkinje fibers/cells. The inner part of the endocardium contains more regularly arranged collagen and elastic fibers than the outer layer. Some myofibroblasts are present in the endocardium which is thicker in the atria than in the ventricles. There is a subendothelial component of the endocardium underneath the endothelium. The component contains fibroblasts, scattered smooth muscle cells, elastic fibers, collagen fibers, and an amorphous ground substance that contains glycoproteins and proteoglycans. The valves of the heart are attached to the cardiac skeleton and consist of chondroid (a material resembling cartilage). The base of each valve is supported by a fibrocollagenous ring. Each valve also has a dense fibrocollagenous central plate that is covered by simple squamous epithelium. Chordae tendonae connect with the valves at the edge of each cusp as well as underneath each cusp at one end and they attach to papillary muscles in the ventricles at the other end. Endocardial endothelium completely covers the papillary muscles, valves, and the chordae tendonae. The junctions between the cusps of each valve are known as commissures. The conducting system of the heart consists of four main components; the sinuatrial node (SA), the atrioventricular node (AV), the bundle of his, and the Purkinje fibers/cells. All the parts of this conducting system are composed of modified cardiac muscle cells. The SA node is located in the right atrium, at the point where the superior vena cava enters. The small muscle fibers of the SA node contain a central nodal artery and desmosomes. The muscle fibers do not contain intercalated discs. The AV node is located in the medial wall, in front of the opening of the coronary sinus and above the tricuspid ring. Its small muscle fibers are more regularly arranged than those of the SA node. The AV node contains a rich nerve and blood supply. The bundle of his has a right (single bundle) and a left (branched bundle) bundle branch located underneath the endocardium. It is histologically similar to the other components of the conducting system. The Purkinje fibers/cells can be found in clusters of about six cells which are located under the endocardium in the ventricles. The cytoplasm of Purkinje fibers appears pale under the microscope and contains many glycogen granules.7 Physiology of the Heart: The principle function of the heart and circulatory system is to provide oxygen and nutrients and to remove metabolic waste products from tissues and organs of the body. The heart is the pump that provides the energy necessary for transporting the blood through the circulatory system in order to facilitate the exchange of oxygen, carbon dioxide, and other metabolites through the thin-walled capillaries. The contraction of the heart produces changes in pressures and flows in the heart chambers and blood vessels. The mechanical events of the cardiac cycle can be divided into four periods; late diastole, atrial systole, ventricular systole, and early diastole.6 In late diastole, the mitral and tricuspid valves are open and the pulmonary and aortic valves are closed. Blood flows into the heart throughout diastole thus filling the atria and ventricles. The rate of filling declines as the ventricles become distended, and the cusps of the atrioventricular valves start to close. The pressure in the ventricles remains low throughout late diastole.8 In atrial systole, contraction of the atria forces additional blood into the ventricles, but approximately 70 percent of the ventricular filling occurs passively during diastole. Contraction of the atrial muscle that surrounds the openings of the superior and inferior vena cava and pulmonary veins, narrows their orifices and the inertia of the blood moving towards the heart tends to keep blood in the heart. However, there is some regurgitation of blood into the veins during atrial systole.2&5 At the start of ventricular systole, the AV valves close. The muscles of the ventricles initially contract relatively little, but intraventricular pressure rises sharply as the muscles squeezes the blood in the ventricle. This period of isovolumetric ventricular contraction lasts about 0.05 seconds until the pressures in the ventricles exceed the pressure in the aorta and in the pulmonary artery, and the aortic and pulmonary valves (semilunar valves) open. During this isovolumetric contraction, the AV valves bulge into the atria, causing a small but sharp rise in atrial pressure. When the semilunar valves open, the phase of ventricular ejection begins. Ejection is initially rapid, but slows down as systole progresses. The intraventricular pressure rises to a maximum and then declines somewhat before ventricular systole ends. Late in systole, the aortic pressure is actually higher than the ventricular pressure, but for a short period, momentum keeps the blood moving forward. The AV valves are pulled down by the contractions of the ventricular muscle, and the atrial pressure drops.5 In early diastole, after the ventricular muscle if fully contracted, the already falling ventricular pressure drops even more rapidly. This is the period known as protodiastole and it lasts about 0.04 seconds. It ends when the momentum of the ejected blood is overcome and the semilunar valves close. After the valves are closed, pressure continues to drop rapidly during the period of isovolumetric relaxation. Isovolumetric relaxation ends when the ventricular pressure falls below the atrial pressure and the AV valves open, thus allowing the ventricles to fill. Again, filling is rapid at first, then slows as the next cardiac contraction approaches. Atrial pressure continues to rise after the end of ventricular systole until the AV valves open, upon which time it drops and slowly rises again until the next atrial systole.6,2,&4 Summary: The heart is arguably the most vital organ the human body possesses. Without the heart, none of the tissues in the body would receive the vital oxygen necessary for them to maintain survival. Heart disease is the number one killer of people in America today. Due to this disturbing fact, it is no wonder such a large percentage of the fellowships granted by the National Institutes of Health go towards heart related illnesses.